In a conventional printer of this type, it is common to provide one electromagnet to engage the clutch for one printing cycle in response to a printing signal, and another electromagnet to operate the hammer impact change-over mechanism according to the size of the printing surface of a selected printing type element. Therefore, a change-over drive arrangement including these electromagnets must be provided that is both intricate in construction and costly to manufacture. These problems are compounded in a printer in which the ribbon lifting mechanism and the hammer operating mechanism are supported on a carrier movable along the printing line of the platen because the mechanisms on the carrier are large in size and weight.
In the hammer operating mechanism of a conventional printer, the energizing force of a first spring member having a large elastic force is applied to a cam follower member which is abutted against a cam member and is operatively coupled to the printing hammer. Where printing is being carried out with a printing type element having a large printing surface, the cam follower member is turned as the cam member rotates so that the printing hammer is operated to strongly strike the type element, i.e., printing is performed with large impact. On the other hand, where printing is carried out with a printing type element having a small printing surface, a switching operation is conducted by means of an electromagnet or the like so that the energizing force of a second spring member having a smaller elastic force is applied to the cam follower. The direction of the energizing force of the second spring member is opposite to that of the energizing force of the first spring member, with the result that the energizing force of the first spring member is decreased. Under this condition, the cam follower member is turned as the cam member rotates, so that the printing hammer is operated to strike the type element less strongly, i.e., printing is achieved with a weaker impact.
In the conventional printer designed as described above, the first spring member must be one having a large elastic force. Accordingly, components operatively coupled to the first spring member must be strong enough to withstand the large elastic force of the first spring member. This requires that the mechanisms of the printer to be heavy.